Long Term Evolution (LTE)
Third-generation mobile systems (3G) based on WCDMA radio-access technology are being deployed on a broad scale all around the world. A first step in enhancing or evolving this technology entails introducing High-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, also referred to as High Speed Uplink Packet Access (HSUPA), giving a radio-access technology that is highly competitive.
In order to be prepared for further increasing user demands and to be competitive against new radio access technologies 3GPP introduced a new mobile communication system which is called Long Term Evolution (LTE). LTE is designed to meet the carrier needs for high speed data and media transport as well as high capacity voice support to the next decade. The ability to provide high bit rates is a key measure for LTE.
The work item (WI) specification on Long-Term Evolution (LTE) called Evolved UMTS Terrestrial Radio Access (UTRA) and UMTS Terrestrial Radio Access Network (UTRAN) is to be finalized as Release 8 (LTE Rel. 8). The LTE system represents efficient packet-based radio access and radio access networks that provide full IP-based functionalities with low latency and low cost. The detailed system requirements are given in. In LTE, scalable multiple transmission bandwidths are specified such as 1.4, 3.0, 5.0, 10.0, 15.0, and 20.0 MHz, in order to achieve flexible system deployment using a given spectrum. In the downlink, Orthogonal Frequency Division Multiplexing (OFDM) based radio access was adopted because of its inherent immunity to multipath interference (MPI) due to a low symbol rate, the use of a cyclic prefix (CP), and its affinity to different transmission bandwidth arrangements. Single-carrier frequency division multiple access (SC-FDMA) based radio access was adopted in the uplink, since provisioning of wide area coverage was prioritized over improvement in the peak data rate considering the restricted transmission power of the user equipment (UE). Many key packet radio access techniques are employed including multiple-input multiple-output (MIMO) channel transmission techniques, and a highly efficient control signaling structure is achieved in LTE Rel. 8.
LTE Architecture
The overall architecture is shown in FIG. 1 and a more detailed representation of the E-UTRAN architecture is given in FIG. 2. The E-UTRAN consists of eNode B, providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the user equipment (UE). The eNode B (eNB) hosts the Physical (PHY), Medium Access Control (MAC), Radio Link Control (RLC), and Packet Data Control Protocol (PDCP) layers that include the functionality of user-plane header-compression and encryption. It also offers Radio Resource Control (RRC) functionality corresponding to the control plane. It performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated uplink QoS, cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of downlink/uplink user plane packet headers. The eNode Bs are interconnected with each other by means of the X2 interface.
The eNode Bs are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME and to the Serving Gateway (SGW) by means of the S1-U. The S1 interface supports a many-to-many relation between MMEs/Serving Gateways and eNode Bs. The SGW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNode B handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and PDN GW). For idle state user equipments, the SGW terminates the downlink data path and triggers paging when downlink data arrives for the user equipment. It manages and stores user equipment contexts, e.g. parameters of the IP bearer service, network internal routing information. It also performs replication of the user traffic in case of lawful interception.
The MME is the key control-node for the LTE access-network. It is responsible for idle mode user equipment tracking and paging procedure including retransmissions. It is involved in the bearer activation/deactivation process and is also responsible for choosing the SGW for a user equipment at the initial attach and at time of intra-LTE handover involving Core Network (CN) node relocation. It is responsible for authenticating the user (by interacting with the HSS). The Non-Access Stratum (NAS) signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to user equipments. It checks the authorization of the user equipment to camp on the service provider's Public Land Mobile Network (PLMN) and enforces user equipment roaming restrictions. The MME is the termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management. Lawful interception of signaling is also supported by the MME. The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. The MME also terminates the Sha interface towards the home HSS for roaming user equipments.
Uplink Access Scheme for LTE
For uplink transmission, power-efficient user-terminal transmission is necessary to maximize coverage. Single-carrier transmission combined with FDMA (Frequency Division Multiple Access) with dynamic bandwidth allocation has been chosen as the evolved UTRA uplink transmission scheme. The main reason for the preference for single-carrier transmission is the lower peak-to-average power ratio (PAPR), compared to multi-carrier signals (OFDMA—Orthogonal Frequency Division Multiple Access), and the corresponding improved power-amplifier efficiency and assumed improved coverage (higher data rates for a given terminal peak power). During each time interval, eNode B assigns users a unique time/frequency resource for transmitting user data thereby ensuring intra-cell orthogonality. An orthogonal access in the uplink promises increased spectral efficiency by eliminating intra-cell interference. Interference due to multipath propagation is handled at the base station (eNode B), aided by insertion of a cyclic prefix in the transmitted signal.
The basic physical resource used for data transmission consists of a frequency resource of size BWgrant during one time interval, e.g. a sub-frame of 0.5 ms, onto which coded information bits are mapped. It should be noted that a sub-frame, also referred to as transmission time interval (TTI), is the smallest time interval for user data transmission. It is however possible to assign a frequency resource BWgrant over a longer time period than one TTI to a user by concatenation of sub-frames.
The frequency resource can either be in a localized or distributed spectrum as illustrated in FIG. 3 and FIG. 4. As can be seen from FIG. 3, localized single-carrier is characterized by the transmitted signal having a continuous spectrum that occupies a part of the total available spectrum. Different symbol rates (corresponding to different data rates) of the transmitted signal imply different bandwidths of a localized single-carrier signal.
On the other hand, as shown in FIG. 4, distributed single-carrier is characterized by the transmitted signal having a non-continuous (“comb-shaped”) spectrum that is distributed over system bandwidth. Note that, although the distributed single-carrier signal is distributed over the system bandwidth, the total amount of occupied spectrum is, in essence, the same as that of localized single-carrier. Furthermore, for higher/lower symbol rate, the number of “comb-fingers” is increased/reduced, while the “bandwidth” of each “comb finger” remains the same.
At first glance, the spectrum in FIG. 4 may give the impression of a multi-carrier signal where each comb-finger corresponds to a “sub-carrier”. However, from the time-domain signal-generation of a distributed single-carrier signal, it should be clear that what is being generated is a true single-carrier signal with a corresponding low peak-to-average power ratio. The key difference between a distributed single-carrier signal versus a multi-carrier signal, such as e.g. OFDM (Orthogonal Frequency Division Multiplex), is that, in the former case, each “sub-carrier” or “comb finger” does not carry a single modulation symbol. Instead each “comb-finger” carries information about all modulation symbols. This creates a dependency between the different comb-fingers that leads to the low-PAPR characteristics. It is the same dependency between the “comb fingers” that leads to a need for equalization unless the channel is frequency-non-selective over the entire transmission bandwidth. In contrast, for OFDM equalization is not needed as long as the channel is frequency-non-selective over the sub-carrier bandwidth.
Distributed transmission can provide a larger frequency diversity gain than localized transmission, while localized transmission more easily allows for channel-dependent scheduling. Note that, in many cases the scheduling decision may decide to give the whole bandwidth to a single user equipment to achieve high data rates.
Uplink Scheduling Scheme for LTE
The uplink scheme allows for both scheduled access, i.e. controlled by eNodeB, and contention-based access.
In case of scheduled access the user equipment is allocated a certain frequency resource for a certain time (i.e. a time/frequency resource) for uplink data transmission. However, some time/frequency resources can be allocated for contention-based access. Within these time/frequency resources, user equipments can transmit without first being scheduled. One scenario where user equipment is making a contention-based access is for example the random access, i.e. when user equipment is performing initial access to a cell or for requesting uplink resources.
For the scheduled access eNodeB scheduler assigns a user a unique frequency/time resource for uplink data transmission. More specifically the scheduler determines                which user equipment(s) that is (are) allowed to transmit,        which physical channel resources (frequency),        Transport format (Transport Block Size (TBS) and Modulation Coding Scheme (MCS)) to be used by the mobile terminal for transmission        
The allocation information is signaled to the user equipment via a scheduling grant, sent on the so-called L1/L2 control channel. For simplicity, this downlink channel is referred to the “uplink grant channel” in the following.
A scheduling grant message (also referred to as an resource assignment herein) contains at least information which part of the frequency band the user equipment is allowed to use, the validity period of the grant, and the transport format the user equipment has to use for the upcoming uplink transmission. The shortest validity period is one sub-frame. Additional information may also be included in the grant message, depending on the selected scheme. Only “per user equipment” grants are used to grant the right to transmit on the Uplink Shared Channel UL-SCH (i.e. there are no “per user equipment per RB” grants). Therefore the user equipment needs to distribute the allocated resources among the radio bearers according to some rules, which will be explained in detail in the next section.
Unlike in HSUPA there is no user equipment based transport format selection. The base station (eNodeB) decides the transport format based on some information, e.g. reported scheduling information and QoS information, and user equipment has to follow the selected transport format. In HSUPA eNodeB assigns the maximum uplink resource and user equipment selects accordingly the actual transport format for the data transmissions.
Uplink data transmissions are only allowed to use the time-frequency resources assigned to the user equipment through the scheduling grant. If the user equipment does not have a valid grant, it is not allowed to transmit any uplink data. Unlike in HSUPA, where each user equipment is always allocated a dedicated channel there is only one uplink data channel shared by multiple users (UL-SCH) for data transmissions.
To request resources, the user equipment transmits a resource request message to the eNodeB. This resources request message could for example contain information on the buffer status, the power status of the user equipment and some Quality of Services (QoS) related information. This information, which will be referred to as scheduling information, allows eNodeB to make an appropriate resource allocation. Throughout the document it's assumed that the buffer status is reported for a group of radio bearers. Of course other configurations for the buffer status reporting are also possible. Since the scheduling of radio resources is the most important function in a shared channel access network for determining Quality of Service, there are a number of requirements that should be fulfilled by the uplink scheduling scheme for LTE in order to allow for an efficient QoS management (see 3GPP RAN WG#2 Tdoc. R2-R2-062606, “QoS operator requirements/use cases for services sharing the same bearer”, by T-Mobile, NTT DoCoMo, Vodafone, Orange, KPN; available at http://www.3gpp.org/ and incorporated herein by reference):                Starvation of low priority services should be avoided        Clear QoS differentiation for radio bearers/services should be supported by the scheduling scheme        The uplink reporting should allow fine granular buffer reports (e.g. per radio bearer or per radio bearer group) in order to allow the eNode B scheduler to identify for which Radio Bearer/service data is to be sent.        It should be possible to make clear QoS differentiation between services of different users        It should be possible to provide a minimum bit-rate per radio bearer        
As can be seen from above list one essential aspect of the LTE scheduling scheme is to provide mechanisms with which the operator can control the partitioning of its aggregate cell capacity between the radio bearers of the different QoS classes. The QoS class of a radio bearer is identified by the QoS profile of the corresponding SAE bearer signaled from serving gateway to eNode B as described before. An operator can then allocate a certain amount of its aggregate cell capacity to the aggregate traffic associated with radio bearers of a certain QoS class.
The main goal of employing this class-based approach is to be able to differentiate the treatment of packets depending on the QoS class they belong to. For example, as the load in a cell increases, it should be possible for an operator to handle this by throttling traffic belonging to a low-priority QoS class. At this stage, the high-priority traffic can still experience a low-loaded situation, since the aggregate resources allocated to this traffic is sufficient to serve it. This should be possible in both uplink and downlink direction.
One benefit of employing this approach is to give the operator full control of the policies that govern the partitioning of the bandwidth. For example, one operator's policy could be to, even at extremely high loads, avoid starvation of traffic belonging to its lowest priority QoS Class. The avoidance of starvation of low priority traffic is one of the main requirements for the uplink scheduling scheme in LTE. In current UMTS Release 6 (HSUPA) scheduling mechanism the absolute prioritization scheme may lead to starvation of low priority applications. E-TFC selection (Enhanced Transport Format Combination selection) is done only in accordance to absolute logical channel priorities, i.e. the transmission of high priority data is maximized, which means that low priority data is possibly starved by high priority data. In order to avoid starvation the eNode B scheduler must have means to control from which radio bearers a user equipment transmits data. This mainly influences the design and use of the scheduling grants transmitted on the L1/L2 control channel in downlink. In the following the details of the uplink rate control procedure in LTE is outlined.
Uplink Rate Control/Logical Channel Prioritization Procedure
For UMTS long term evolution (LTE) uplink transmissions there is a desire that starvation be avoided and greater flexibility in resource assignment between bearers be possible, whilst retaining the per user equipment, rather than per user equipment bearer, resource allocation.
The user equipment has an uplink rate control function which manages the sharing of uplink resources between radio bearers. This uplink rate control function is also referred to as logical channel prioritization procedure in the following. The Logical Channel Prioritization (LCP) procedure is applied when a new transmission is performed, i.e. a transport block needs to be generated. One proposal for assigning capacity has been to assign resources to each bearer, in priority order, until each has received an allocation equivalent to the minimum data rate for that bearer, after which any additional capacity is assigned to bearers in, for example, priority order.
As will become evident from the description of the LCP procedure given below, the implementation of the LCP procedure residing in the user equipment is based on the token bucket model, which is well known in the IP world. The basic functionality of this model is as follows. Periodically and at a given rate, a token which represents the right to transmit a quantity of data is added to the bucket. When the user equipment is granted resources, it is allowed to transmit data up to the amount represented by the number of tokens in the bucket. When transmitting data the user equipment removes the number of tokens equivalent to the quantity of transmitted data. In case the bucket is full, any further tokens are discarded. For the addition of tokens it could be assumed that the period of the repetition of this process would be every TTI, but it could be easily lengthened such that a token is only added every second. Basically instead of every 1 ms a token is added to the bucket, 1000 tokens could be added every second.
In the following the logical channel prioritization procedure used in LTE Rel. 8 is described (see for further details: 3GPP TS 36.321, “Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access Control (MAC) protocol specification”, version 8.5, available at http://www.3gpp.org and incorporated herein by reference).
RRC controls the scheduling of uplink data by signalling for each logical channel: priority where an increasing priority value indicates a lower priority level, prioritisedBitRate which sets the Prioritized Bit Rate (PBR), bucketSizeDuration which sets the Bucket Size Duration (BSD). The idea behind prioritized bit rate is to support for each bearer, including low priority non-GBR bearers, a minimum bit rate in order to avoid a potential starvation. Each bearer should at least get enough resources in order to achieve the prioritized bit rate (PRB).
The UE shall maintain a variable Bj for each logical channel j. Bj shall be initialized to zero when the related logical channel is established, and incremented by the product PBR×TTI duration for each TTI, where PBR is Prioritized Bit Rate of logical channel j. However, the value of Bj can never exceed the bucket size and if the value of Bj is larger than the bucket size of logical channel j, it shall be set to the bucket size. The bucket size of a logical channel is equal to PBR×BSD, where PBR and BSD are configured by upper layers.
The UE shall perform the following Logical Channel Prioritization procedure when a new transmission is performed. The uplink rate control function ensures that the UE serves its radio bearer(s) in the following sequence:
1. All the logical channel(s) in decreasing priority order up to their configured PBR (according the number of tokens in the bucket which is denoted by Bj);
2. If any resources remain, all the logical channels are served in a strict decreasing priority order (regardless of the value of Bj) until either the data for that logical channel or the UL grant is exhausted, whichever comes first. Logical channels configured with equal priority should be served equally.
In case the PBRs are all set to zero, the first step is skipped and the logical channel(s) are served in strict priority order: the UE maximizes the transmission of higher priority data.
The UE shall also follow the rules below during the scheduling procedures above:                the UE should not segment an RLC SDU (or partially transmitted SDU or retransmitted RLC PDU) if the whole SDU (or partially transmitted SDU or retransmitted RLC PDU) fits into the remaining resources;        if the UE segments an RLC SDU from the logical channel, it shall maximize the size of the segment to fill the grant as much as possible;        UE should maximize the transmission of data.        
Even though for LTE Rel. 8 only a Prioritized Bit Rate (PBR) is used within the LCP procedure there could be also further enhancements in future releases. For example similar to the PBR, also a maximum bit rate (MBR) per GBR bearer and an aggregated maximum bit rate (AMBR) for all Non-GBR bearers could be provided to the user equipment. The MBR denotes bit rates of traffic per bearer while AMBR denotes a bit rate of traffic per group of bearers. AMBR applies to all Non-GBR SAE Bearers of a user equipment. GBR SAE Bearers are outside the scope of AMBR. Multiple SAE Non-GBR bearers can share the same AMBR. That is, each of those SAE bearers could potentially utilize the entire AMBR, e.g. when the other SAE bearers do not carry any traffic. The AMBR limits the aggregated bit rate that can be expected to be provided by the Non-GBR SAE bearers sharing the AMBR.
HARQ Protocol Operation for Unicast Data Transmissions
A common technique for error detection and correction in packet transmission systems over unreliable channels is called hybrid Automatic Repeat request (HARQ). Hybrid ARQ is a combination of Forward Error Correction (FEC) and ARQ.
If a FEC encoded packet is transmitted and the receiver fails to decode the packet correctly (errors are usually checked by a CRC (Cyclic Redundancy Check)), the receiver requests a retransmission of the packet
In LTE there are two levels of re-transmissions for providing reliability, namely, HARQ at the MAC layer and outer ARQ at the RLC layer. The outer ARQ is required to handle residual errors that are not corrected by HARQ that is kept simple by the use of a single bit error-feedback mechanism, i.e. ACK/NACK. An N-process stop-and-wait HARQ is employed that has asynchronous re-transmissions in the downlink and synchronous re-transmissions in the uplink. Synchronous HARQ means that the re-transmissions of HARQ blocks occur at pre-defined periodic intervals. Hence, no explicit signaling is required to indicate to the receiver the retransmission schedule. Asynchronous HARQ offers the flexibility of scheduling re-transmissions based on air interface conditions. In this case some identification of the HARQ process needs to be signaled in order to allow for a correct combing and protocol operation. In 3GPP, HARQ operations with eight processes is used in LTE Rel. 8. The HARQ protocol operation for Downlink data transmission will be similar or even identical to HSDPA.
In uplink HARQ protocol operation there are two different options on how to schedule a retransmission. Retransmissions are either scheduled by a NACK, synchronous non-adaptive retransmission, or explicitly scheduled by a PDCCH, synchronous adaptive retransmissions. In case of a synchronous non-adaptive retransmission the retransmission will use the same parameters as the previous uplink transmission, i.e. the retransmission will be signaled on the same physical channel resources respectively uses the same modulation scheme. Since synchronous adaptive retransmissions are explicitly scheduled via PDCCH, the eNode B has the possibility to change certain parameters for the retransmission. A retransmission could be for example scheduled on a different frequency resource in order to avoid fragmentation in the uplink, or the eNode B could change the modulation scheme or alternatively indicate user equipment what redundancy version to use for the retransmission. It should be noted that the HARQ feedback (ACK/NACK) and PDCCH signaling occurs at the same timing. Therefore user equipment only needs to check once whether a synchronous non-adaptive retransmission is triggered, only NACK is received, or whether the eNode B requests a synchronous adaptive retransmission, i.e. PDCCH is signaled.
L1/L2 Control Signaling
In order to inform the scheduled users about their allocation status, transport format and other data related information (e.g. HARQ), L1/L2 control signaling needs to be transmitted on the downlink along with the data. The control signaling needs to be multiplexed with the downlink data in a sub-frame (assuming that the user allocation can change from sub-frame to sub-frame). Here, it should be noted, that user allocation might also be performed on a TTI (Transmission Time Interval) basis, where the TTI length is a multiple of the sub-frames. The TTI length may be fixed in a service area for all users, may be different for different users, or may even by dynamic for each user. Generally, then the L1/2 control signaling needs only be transmitted once per TTI. The L1/L2 control signaling is transmitted on the Physical Downlink Control Channel (PDCCH). It should be noted that assignments for uplink data transmissions, uplink grants, are also transmitted on the PDCCH.
Generally, the PDCCH information sent on the L1/L2 control signaling may be separated into the Shared Control Information (SCI) and Dedicated Control Information (DCI).
Shared Control Information (SCI)
Shared Control Information (SCI) carries so-called Cat 1 information. The SCI part of the L1/L2 control signaling contains information related to the resource allocation (indication). The SCI typically contains the following information:                User identity, indicating the user which is allocated        RB allocation information, indicating the resources (Resource Blocks, RBs) on which a user is allocated. Note, that the number of RBs on which a user is allocated can be dynamic.        Duration of assignment (optional) if an assignment over multiple sub-frames (or TTIs) is possible        
Depending on the setup of other channels and the setup of the Dedicated Control Information (DCI), the SCI may additionally contain information such as ACK/NACK for uplink transmission, uplink scheduling information, information on the DCI (resource, MCS, etc.).
Dedicated Control Information (DCI)
Dedicated Control Information (DCI) carries the so-called Cat 2/3 information. The DCI part of the L1/L2 control signaling contains information related to the transmission format (Cat 2) of the data transmitted to a scheduled user indicated by Cat 1. Moreover, in case of application of (hybrid) ARQ it carries HARQ (Cat 3) information. The DCI needs only to be decoded by the user scheduled according to Cat 1. The DCI typically contains information on:                Cat 2: Modulation scheme, transport-block (payload) size (or coding rate), MIMO related information, etc. Note, either the transport-block (or payload size) or the code rate can be signaled. In any case these parameters can be calculated from each other by using the modulation scheme information and the resource information (number of allocated RBs).        Cat 3: HARQ related information, e.g. hybrid ARQ process number, redundancy version, retransmission sequence number        
L1/L2 control signaling information for Downlink Data Transmission
Along with the downlink packet data transmission, L1/L2 control signaling is transmitted on a separate physical channel (PDCCH). This L1/L2 control signaling typically contains information on:                The physical channel resource(s) on which the data is transmitted (e.g. subcarriers or subcarrier blocks in case of OFDM, codes in case of CDMA). This information allows the user equipment (receiver) to identify the resources on which the data is transmitted.        The transport Format, which is used for the transmission. This can be the transport block size of the data (payload size, information bits size), the MCS (Modulation and Coding Scheme) level, the Spectral Efficiency, the code rate, etc. This information (usually together with the resource allocation) allows the user equipment (receiver) to identify the information bit size, the modulation scheme and the code rate in order to start the demodulation, the de-rate-matching and the decoding process. In some cases the modulation scheme maybe signaled explicitly.        HARQ information:                    Process number: Allows the user equipment to identify the HARQ process on which the data is mapped.            Sequence number or new data indicator: Allows the user equipment to identify if the transmission is a new packet or a retransmitted packet.            Redundancy and/or constellation version: Tells the user equipment, which hybrid ARQ redundancy version is used (required for de-rate-matching) and/or which modulation constellation version is used (required for demodulation)                        user equipment Identity (user equipment ID): Tells for which user equipment the L1/L2 control signaling is intended for. In typical implementations this information is used to mask the CRC of the L1/L2 control signaling in order to prevent other user equipments to read this information.        
L1/L2 Control Signaling Information for Uplink Data Transmission
To enable an uplink packet data transmission, L1/L2 control signaling is transmitted on the downlink (PDCCH) to tell the user equipment about the transmission details. This L1/L2 control signaling typically contains information on:                The physical channel resource(s) on which the user equipment should transmit the data (e.g. subcarriers or subcarrier blocks in case of OFDM, codes in case of CDMA).        The transport format, the user equipment should use for the transmission. This can be the transport block size of the data (payload size, information bits size), the MCS (Modulation and Coding Scheme) level, the Spectral Efficiency, the code rate, etc. This information (usually together with the resource allocation) allows the user equipment (transmitter) to pick the information bit size, the modulation scheme and the code rate in order to start the modulation, the rate-matching and the encoding process. In some cases the modulation scheme maybe signaled explicitly.        Hybrid ARQ information:                    Process number: Tells the user equipment from which hybrid ARQ process it should pick the data.            Sequence number or new data indicator: Tells the user equipment to transmit a new packet or to retransmit a packet.            Redundancy and/or constellation version: Tells the user equipment, which hybrid ARQ redundancy version to use (required for rate-matching) and/or which modulation constellation version to use (required for modulation).                        user equipment Identity (user equipment ID): Tells which user equipment should transmit data. In typical implementations this information is used to mask the CRC of the L1/L2 control signaling in order to prevent other user equipments to read this information.        
There are several different flavors how to exactly transmit the information pieces mentioned above. Moreover, the L1/L2 control information may also contain additional information or may omit some of the information. E.g.:                HARQ process number may not be needed in case of a synchronous HARQ protocol.        A redundancy and/or constellation version may not be needed if Chase Combining is used (always the same redundancy and/or constellation version) or if the sequence of redundancy and/or constellation versions is pre defined.        Power control information may be additionally included in the control signaling.        MIMO related control information, such as e.g. pre-coding, may be additionally included in the control signaling.        In case of multi-codeword MIMO transmission transport format and/or HARQ information for multiple code words may be included.        
For uplink resource assignments (PUSCH) signaled on PDCCH in LTE, the L1/L2 control information does not contain a HARQ process number, since a synchronous HARQ protocol is employed for LTE uplink. The HARQ process to be used for an uplink transmission is given by the timing. Furthermore it should be noted that the redundancy version (RV) information is jointly encoded with the transport format information, i.e. the RV info is embedded in the transport format (TF) field. The TF respectively MCS field has for example a size of 5 bits, which corresponds to 32 entries. 3 TF/MCS table entries are reserved for indicating RVs 1, 2 or 3. The remaining MCS table entries are used to signal the MCS level (TBS) implicitly indicating RV0. The size of the CRC field of the PDCCH is 16 bits.
For downlink assignments (PDSCH) signaled on PDCCH in LTE the Redundancy Version (RV) is signaled separately in a two-bit field. Furthermore the modulation order information is jointly encoded with the transport format information. Similar to the uplink case there is 5 bit MCS field signaled on PDCCH. Three of the entries are reserved to signal an explicit modulation order, providing no Transport format (Transport block) info. For the remaining 29 entries modulation order and Transport block size info are signaled.
Uplink Power Control
Uplink transmission power control in a mobile communication system serves an important purpose: it balances the need for sufficient transmitted energy per bit to achieve the required Quality-of-Service (QoS), against the needs to minimize interference to other users of the system and to maximize the battery life of the mobile terminal. In achieving this purpose, the role of the Power Control (PC) becomes decisive to provide the required SINR (Signal to Interference Noise Ratio) while controlling at the same time the interference caused to neighboring cells. The idea of classic PC schemes in uplink is that all users are received with the same SINR, which is known as full compensation. As an alternative, 3GPP has adopted for LTE the use of Fractional Power Control (FPC). This new functionality makes users with a higher path-loss operate at a lower SINR requirement so that they will more likely generate less interference to neighboring cells.
Detailed power control formulae are specified in LTE for the Physical Uplink Shared Channel (PUSCH), Physical Uplink Control Channel (PUCCH) and the Sounding Reference Signals (SRSs) (see section 5.1 of 3GPP TS 36.213, “Physical layer procedures (Release 8)”, version 8.6.0, available at http://www.3gpp.org). The respective power control formula for each of these uplink signals follows the same basic principles. They can be considered as a summation of two main terms: a basic open-loop operating point derived from static or semi-static parameters signaled by the eNodeB, and a dynamic offset updated from sub-frame to sub-frame.
The basic open-loop operating point for the transmit power per resource block depends on a number of factors including the inter-cell interference and cell load. It can be further broken down into two components, a semi-static base level P0, further comprised of a common power level for all user equipments (UEs) in the cell (measured in dBm) and a UE-specific offset, and an open-loop path-loss compensation component. The dynamic offset part of the power per resource block can also be further broken down into two components, a component dependent on the Modulation and Coding Scheme (MCS) and explicit Transmitter Power Control (TPC) commands.
The MCS-dependent component (referred to in the LTE specifications as ΔTF, where TF is short for Transport Format) allows the transmitted power per RB to be adapted according to the transmitted information data rate.
The other component of the dynamic offset is the UE-specific TPC commands. These can operate in two different modes:                accumulative TPC commands (available for PUSCH, PUCCH and SRS) and        absolute TPC commands (available for PUSCH only).        
For the PUSCH, the switch between these two modes is configured semi-statically for each user equipment by RRC signaling—i.e. the mode cannot be changed dynamically. With the accumulative TPC commands, each TPC command signals a power step relative to the previous level.
Formula (1) below shows the user equipment transmit power in dBm for the PUSCH:PPUSCH=min └PMAX,10·log10 M+P0_PUSCH+α·PL+ΔMCS+f(Δi)┘  (1)where:                PMAX is the maximum available transmit power of the user equipment, which is depending on the user equipment class and configuration by the network        M is the number of allocated physical resource blocks (PRBs).        PL is the user equipment path loss derived at the UE-based on RSRP (Reference Signal Received Power) measurement and signaled RS (Reference Symbol) eNodeB transmission power.        ΔMCS is an MCS-dependent power offset set by the eNodeB.        P0_PUSCH is a UE-specific parameter (partially broadcasted and partially signaled using RRC).        α is cell-specific parameter (broadcasted on BCH).        Δi are closed loop PC commands signaled from the eNodeB to the user equipment        function ƒ( ) indicates whether closed loop commands are relative accumulative or absolute. The function ƒ( ) is signaled to the user equipment via higher layers.Further Advancements for LTE (LTE-A)        
The frequency spectrum for IMT-Advanced was decided at the World Radiocommunication Conference 2007 (WRC-07). Although the overall frequency spectrum for IMT-Advanced was decided, the actual available frequency bandwidth is different according to each region or country. Following the decision on the available frequency spectrum outline, however, standardization of a radio interface started in the 3rd Generation Partnership Project (3GPP). At the 3GPP TSG RAN #39 meeting, the Study Item description on “Further Advancements for E-UTRA (LTE-Advanced)” was approved. The study item covers technology components to be considered for the evolution of E-UTRA, e.g. to fulfill the requirements on IMT-Advanced. Two major technology components which are currently under consideration for LTE-Advanced (LTE-A for short) are described in the following.
LTE-A Support of Wider Bandwidth
Carrier aggregation, where two or more component carriers are aggregated, is considered for LTE-A in order to support wider transmission bandwidths e.g. up to 100 MHz and for spectrum aggregation.
A terminal may simultaneously receive or transmit on one or multiple component carriers depending on its capabilities:                An LTE-A terminal with reception and/or transmission capabilities for carrier aggregation can simultaneously receive and/or transmit on multiple component carriers. There is one Transport Block (in absence of spatial multiplexing) and one HARQ entity per component carrier.        An LTE Rel. 8 terminal can receive and transmit on a single component carrier only, provided that the structure of the component carrier follows the Rel. 8 specifications.        
It shall be possible to configure all component carriers LTE Rel. 8 compatible, at least when the aggregated numbers of component carriers in the uplink and the downlink are same. Consideration of non-backward-compatible configurations of LTE-A component carriers is not precluded
At present, LTE-A supports carrier aggregation for both contiguous and non-contiguous component carriers with each component carrier limited to a maximum of 110 Resource Blocks (RBs) in the frequency domain, using the LTE Rel. 8 numerology. It is possible to configure a user equipment to aggregate a different number of component carriers originating from the same eNodeB. Please note that component carriers originating from the same eNodeB do no necessarily need to provide the same coverage.
Furthermore, a user equipment may be configured with different bandwidths in the uplink and the downlink:                The number of downlink component carriers that can be configured depends on the downlink aggregation capability of the user equipment;        The number of uplink component carriers that can be configured depends on the uplink aggregation capability of the user equipment;        It is not possible to configure a user equipment with more uplink component carriers than downlink component carriers;        In typical TDD deployments, the number of component carriers and the bandwidth of each component carrier in uplink and downlink is the same.        
The spacing between centre frequencies of contiguously aggregated component carriers is a multiple of 300 kHz. This is in order to be compatible with the 100 kHz frequency raster of LTE Rel. 8 and at the same time preserve orthogonality of the subcarriers with 15 kHz spacing. Depending on the aggregation scenario, the n×300 kHz spacing can be facilitated by insertion of a low number of unused subcarriers between contiguous component carriers.
The nature of the aggregation of multiple carriers is only exposed up to the MAC layer. For uplink and for downlink there is one HARQ entity required in MAC for each aggregated component carrier. There is (in the absence of Single User—Multiple Input Multiple Output (SU-MIMO) for uplink) at most one transport block per component carrier. A transport block and its potential HARQ retransmissions need to be mapped on the same component carrier. The Layer 2 structure with configured carrier aggregation is shown in FIG. 5 and FIG. 6 for the downlink and uplink respectively.
When carrier aggregation is configured, the user equipment has only one RRC connection with the network. At RRC connection establishment/re-establishment, one cell provides the security input (one ECGI, one PCI and one ARFCN) and the non-access stratum (NAS) mobility information (e.g. tracking area identifier (TAI)), similar to LTE Rel. 8. After RRC connection establishment/re-establishment, the component carrier corresponding to that cell is referred to as the Downlink Primary Component Carrier (DL PCC) in the downlink. There is always only one DL PCC and one UL PCC configured per user equipment in connected mode. Within the configured set of component carriers, other component carriers are referred to as Secondary Component Carriers (SCCs).
The characteristics of the DL PCC and UL PCC are:                The UL PCC is used for transmission of Layer 1 (L1) uplink control information;        The DL PCC cannot be de-activated;        Re-establishment of the DL PCC is triggered when the DL PCC experiences Radio Link Failure (RLF), but not when DL SCCs experience RLF;        The DL PCC cell can change with handover;        NAS information is taken from the DL PCC cell.        
The reconfiguration, addition and removal of component carriers can be performed by RRC signaling. At intra-LTE handover, RRC can also add, remove, or reconfigure component carriers for usage in the target cell. When adding a new component carrier, dedicated RRC signaling is used for sending component carrier's system information which is necessary for component carrier transmission/reception (similarly as in LTE Rel.8 for handover).
When carrier aggregation is configured, a user equipment may be scheduled over multiple component carriers simultaneously but at most one random access procedure shall be ongoing at any time. Cross-carrier scheduling allows the PDCCH of a component carrier to schedule resources on another component carrier. For this purpose a component carrier identification field is introduced in the respective DCI formats (called “CIF”). A linking between uplink and downlink component carriers allows identifying the uplink component carrier for which the grant applies when there is no-cross-carrier scheduling. The linkage of downlink component carriers to uplink component carriers does not necessarily need to be one to one. In other words, more than one downlink component carrier can link to the same uplink component carrier. At the same time, a downlink component carrier can only link to one uplink component carrier.
(De)Activation of a Component Carrier and DRX Operation
In carrier aggregation, whenever a user equipment is configured with only one component carrier, LTE Rel. 8 DRX operation applies. In other cases, the same DRX operation applies to all configured and activated component carriers (i.e. identical active time for PDCCH monitoring). When in active time, any component carrier may always schedule PDSCH on any other configured and activated component carrier.
To enable reasonable UE battery consumption when carrier aggregation is configured, a component carrier activation/deactivation mechanism for downlink SCCs is introduced (i.e. activation/deactivation does not apply to the PCC). When a downlink SCC is not active, the UE does not need to receive the corresponding PDCCH or PDSCH, nor is it required to perform CQI measurements. Conversely, when a downlink SCC is active, the user equipment should receive the PDSCH and PDCCH (if present), and is expected to be able to perform CQI measurements. In the uplink however, a user equipment is always required to be able to transmit on the PUSCH on any configured uplink component carrier when scheduled on the corresponding PDCCH (i.e. there is no explicit activation of uplink component carriers).
Other details of the activation/deactivation mechanism for SCCs are:                Explicit activation of DL SCCs is done by MAC signaling;        Explicit deactivation of DL SCCs is done by MAC signaling;        Implicit deactivation of DL SCCs is also possible;        DL SCCs can be activated and deactivated individually, and a single activation/deactivation command can activate/deactivate a subset of the configured DL SCCs;        SCCs added to the set of configured CCs are initially “deactivated”.Timing Advance        
As already mentioned above, for the uplink transmission scheme of 3GPP LTE single-carrier frequency division multiple access (SC-FDMA) was chosen to achieve an orthogonal multiple-access in time and frequency between the different user equipments transmitting in the uplink.
Uplink orthogonality is maintained by ensuring that the transmissions from different user equipments in a cell are time-aligned at the receiver of the eNodeB. This avoids intra-cell interference occurring, both between user equipments assigned to transmit in consecutive sub-frames and between user equipments transmitting on adjacent subcarriers. Time alignment of the uplink transmissions is achieved by applying a timing advance at the user equipment's transmitter, relative to the received downlink timing as exemplified in FIG. 7. The main role of this is to counteract differing propagation delays between different user equipments.
Initial Timing Advance Procedure
When user equipment is synchronized to the downlink transmissions received from eNodeB, the initial timing advance is set by means of the random access procedure as described below. The user equipment transmits a random access preamble based on which the eNodeB can estimate the uplink timing. The eNodeB responds with an 11-bit initial timing advance command contained within the Random Access Response (RAR) message. This allows the timing advance to be configured by the eNodeB with a granularity of 0.52 μs from 0 up to a maximum of 0.67 ms.
Additional information on the control of the uplink timing and timing advance on 3GPP LTE (Release 8/9) can be found in chapter 20.2 of Stefania Sesia, Issam Toufik and Matthew Baker, “LTE—The UMTS Long Term Evolution: From Theory to Practice”, John Wiley & Sons, Ltd. 2009, which is incorporated herein by reference.
Updates of the Timing Advance
Once the timing advance has been first set for each user equipment, the timing advance is updated from time to time to counteract changes in the arrival time of the uplink signals at the eNodeB. In deriving the timing advance update commands, the eNodeB may measure any uplink signal which is useful. The details of the uplink timing measurements at the eNodeB are not specified, but left to the implementation of the eNodeB.
The timing advance update commands are generated at the Medium Access Control (MAC) layer in the eNodeB and transmitted to the user equipment as MAC control elements which may be multiplexed together with data on the Physical Downlink Shared Channel (PDSCH). Like the initial timing advance command in the response to the Random Access Channel (RACH) preamble, the update commands have a granularity of 0.52 μs. The range of the update commands is ±16 μs, allowing a step change in uplink timing equivalent to the length of the extended cyclic prefix. They would typically not be sent more frequently than about every 2 seconds. In practice, fast updates are unlikely to be necessary, as even for a user equipment moving at 500 km/h the change in round-trip path length is not more than 278 m/s, corresponding to a change in round-trip time of 0.93 μs/s.
The eNodeB balances the overhead of sending regular timing update commands to all the UEs in the cell against a UE's ability to transmit quickly when data arrives in its transmit buffer. The eNodeB therefore configures a timer for each user equipment, which the user equipment restarts each time a timing advance update is received. In case the user equipment does not receive another timing advance update before the timer expires, it must then consider that it has lost uplink synchronization (see also section 5.2 of 3GPP TS 36.321, “Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access Control (MAC) protocol specification”, version 8.9.0, available at http://www.3gpp.org and incorporated herein by reference).
In such a case, in order to avoid the risk of generating interference to uplink transmissions from other user equipments, the UE is not permitted to make another uplink transmission of any sort and needs to revert to the initial timing alignment procedure in order to restore the uplink timing.
Random Access Procedure
A mobile terminal in LTE can only be scheduled for uplink transmission, if its uplink transmission is time synchronized. Therefore the Random Access (RACH) procedure plays an important role as an interface between non-synchronized mobile terminals (UEs) and the orthogonal transmission of the uplink radio access.
Essentially the Random Access in LTE is used to achieve uplink time synchronization for a user equipment which either has not yet acquired, or has lost, its uplink synchronization. Once a user equipment has achieved uplink synchronization the eNodeB can schedule uplink transmission resources for it. The following scenarios are therefore relevant for random access:                A user equipment in RRC_CONNECTED state, but not uplink-synchronized, wishing to send new uplink data or control information        A user equipment in RRC_CONNECTED state, but not uplink-synchronized, required to receive downlink data, and therefore to transmit corresponding HARQ feedback, i.e. ACK/NACK, in the uplink. This scenario is also referred to as Downlink data arrival        A user equipment in RRC_CONNECTED state, handing over from its current serving cell to a new target cell; in order to achieve uplink time-synchronization in the target cell Random Access procedure is performed        A transition from RRC13 IDLE state to RRC_CONNECTED, for example for initial access or tracking area updates        Recovering from radio link failure, i.e. RRC connection re-establishment        
There is one more additional case, where user equipment performs random access procedure, even though user equipment is time-synchronized. In this scenario the user equipment uses the random access procedure in order to send a scheduling request, i.e. uplink buffer status report, to its eNodeB, in case it does not have any other uplink resource allocated in which to send the scheduling request, i.e. dedicated scheduling request (D-SR) channel is not configured.
LTE offers two types of random access procedures that allow access to be either contention based, i.e. implying an inherent risk of collision, or contention-free (non-contention based). It should be noted that contention-based random access can be applied for all six scenarios listed above, whereas a non-contention based random access procedure can only be applied for the downlink data arrival and handover scenario.
In the following the contention based random access procedure is being described in more detail with respect to FIG. 8. A detailed description of the random access procedure can be also found in 3GPP 36.321, section 5.1.
FIG. 8 shows the contention based RACH procedure of LTE. This procedure consists of four “steps”. First, the user equipment transmits 801 a random access preamble on the Physical Random Access Channel (PRACH) to the eNodeB. The preamble is selected by user equipment from the set of available random access preambles reserved by eNodeB for contention based access. In LTE, there are 64 preambles per cell which can be used for contention-free as well as contention based random access. The set of contention based preambles can be further subdivided into two groups, so that the choice of preamble can carry one bit of information to indicate information relating to the amount of transmission resources needed to transmit for the first scheduled transmission, which is referred to as msg3 in TS36.321 (see step 703). The system information broadcasted in the cell contain the information which signatures (preambles) are in each of the two subgroups as well as the meaning of each subgroup. The user equipment randomly selects one preamble from the subgroup corresponding to the size of transmission resource needed for message 3 transmission.
After eNodeB has detected a RACH preamble, it sends 802 a Random Access Response (RAR) message on the PDSCH (Physical Downlink Shared Channel) addressed on the PDCCH with the (Random Access) RA-RNTI identifying the time-frequency slot in which the preamble was detected. If multiple user equipments transmitted the same RACH preamble in the same PRACH resource, which is also referred to as collision, they would receive the same random access response.
The RAR message conveys the detected RACH preamble, a timing alignment command (TA command) for synchronization of subsequent uplink transmissions, an initial uplink resource assignment (grant) for the transmission of the first scheduled transmission (see step 803) and an assignment of a Temporary Cell Radio Network Temporary Identifier (T-CRNTI). This T-CRNTI is used by eNodeB in order to address the mobile(s) whose RACH preamble were detected until RACH procedure is finished, since the “real” identity of the mobile is at this point not yet known by eNodeB.
Furthermore the RAR message can also contain a so-called back-off indicator, which the eNodeB can set to instruct the user equipment to back off for a period of time before retrying a random access attempt. The user equipment monitors the PDCCH for reception of random access response within a given time window, which is configured by the eNodeB. In case user equipment doesn't receive a random access response within the configured time window, it retransmits the preamble at the next PRACH opportunity considering a potentially back off period.
In response to the RAR message received from the eNodeB, the user equipment transmits 803 the first scheduled uplink transmission on the resources assigned by the grant within the random access response. This scheduled uplink transmission conveys the actual random access procedure message like for example RRC connection request, tracking area update or buffer status report. Furthermore it includes either the C-RNTI for user equipments in RRC_CONNECTED mode or the unique 48-bit user equipment identity if the user equipments are in RRC13 IDLE mode. In case of a preamble collision having occurred, i.e. multiple user equipments have sent the same preamble on the same PRACH resource, the colliding user equipments will receive the same T-CRNTI within the random access response and will also collide in the same uplink resources when transmitting 803 their scheduled transmission. This may result in interference that no transmission from a colliding user equipment can be decoded at the eNodeB, and the user equipments will restart the random access procedure after having reached maximum number of retransmission for their scheduled transmission. In case the scheduled transmission from one user equipment is successfully decoded by eNodeB, the contention remains unsolved for the other user equipments.
For resolution of this type of contention, the eNode B sends 804 a contention resolution message addressed to the C-RNTI or Temporary C-RNTI, and, in the latter case, echoes the 48-bit user equipment identity contained the scheduled transmission. It supports HARQ. In case of collision followed by a successful decoding of the message sent in step 803, the HARQ feedback (ACK) is only transmitted by the user equipment which detects its own identity, either C-RNTI or unique user equipment ID. Other UEs understand that there was a collision at step 1 and can quickly exit the current RACH procedure and starts another one.
FIG. 9 is illustrating the contention-free random access procedure of 3GPP LTE Rel. 8/9. In comparison to the contention based random access procedure, the contention-free random access procedure is simplified. The eNodeB provides 901 the user equipment with the preamble to use for random access so that there is no risk of collisions, i.e. multiple user equipment transmitting the same preamble. Accordingly, the user equipment is sending 902 the preamble which was signaled by eNodeB in the uplink on a PRACH resource. Since the case that multiple UEs are sending the same preamble is avoided for a contention-free random access, no contention resolution is necessary, which in turn implies that step 804 of the contention based procedure shown in FIG. 8 can be omitted. Essentially a contention-free random access procedure is finished after having successfully received the random access response.
Timing Advance and Component Carrier Aggregation in the Uplink
In currents specifications of the 3GPP standards the user equipment only maintains one timing advance value and applies this to uplink transmissions on all aggregated component carriers. When component carriers are aggregated from different bands, they can experience different interference and coverage characteristics.
Furthermore the deployment of technologies like Frequency Selective Repeaters (FSR) as shown for example in FIG. 11 and Remote Radio Heads (RRH) as shown for example in FIG. 12 will cause different interference and propagation scenarios for the aggregated component carriers. This leads to the need of introducing more than one timing advance within one user equipment.
This leads to the need of introducing more than one timing advance within one UE. There might be a separate timing advance for each aggregated component carrier. Another option is that component carriers that stem from the same location and hence all experience a similar propagation delay are grouped into timing advance groups (TA groups). For each group a separate timing advanced is maintained.
Discussions were already held in 3GPP on this problem but a single timing advance for all aggregated uplink component carriers is regarded as sufficient, since current specifications up to 3GPP LTE-A Rel. 10 support only carrier aggregation of carriers from the same frequency band.
Accordingly, prioritization of different types of uplink transmissions on a plurality of component carriers during a same transmission time interval (TTI) need to be considered. For example when a user equipment (UE) is in power limited state, rules need to determine which uplink transmission should receive the available power.